What is true about most of the world's kimberlite pipes?

Most of the world's kimberlite pipes do not contain diamonds in profitable quantities. While kimberlite pipes are known for being a primary source of diamonds, the reality is that only a small percentage of these pipes actually yield diamonds that can be mined economically. The geological conditions that lead to the formation of kimberlite pipes can produce a variety of materials, but the presence of diamonds does not guarantee their economic viability. They must be in sufficient quantity and of a quality that makes extraction worthwhile, which is not the case for most of these pipes.

The other options suggest more definitive or limited attributes regarding kimberlite pipes. For example, while many kimberlite pipes exist in Africa, they are not exclusive to that continent, making the assertion about geographic limitation inaccurate. Additionally, the notion that they only contain small diamonds or that all contain diamonds in profitable quantities is misleading; the variable quality and sizes of diamonds found in these pipes further complicate the idea of profitability. Therefore, option B accurately captures the general truth about the majority of kimberlite pipes.